Odd-even chain length-dependent order in pH-switchable self-assembled layers

The reversible self-assembly of a series of bipolar amphiphiles, alpha,omega-bis(3- or 4-amidinophenoxy)alkanes (chain length n = 5-12), on mercaptoalkanoic acid-functionalized gold surfaces (chain length n = 10, 11, 14, 15) has been studied by in-situ ellipsometry, IR reflection absorption spectroscopy (IRAS), and atomic force microscopy (AFM). The layer order, amphiphile orientation, and tendency to form bilayers depends on the position of the amidine substituent, the alkyl chain length of both the amidine amphiphile and the underlying acid self-assembled monolayer (SAM), and whether the amidine alkyl chain contained an even or odd number of methylene groups. Thus, para-substituted bisbenzamidines containing more than six methylene groups (n>6) and with an odd number (n = 7, 9, 11) tended to form bilayered structures, whereas those containing an even number formed monolayers when adsorbed on SAMs of the long-chain acids (n = 14, 15). This behavior also correlated with the average tilt angle of the benzene moieties of the amphiphiles, as estimated by IRAS. The odd-numbered chains gave lower tilt angles than the even-numbered ones, and a possible model that accounts for these results is proposed. IRAS also revealed a higher order of the odd-numbered chains and an increasing hydrogen-bonding contribution with increasing chain length. Additional evidence for the proposed bilayered assemblies and their reversibility was obtained by AFM. Images obtained from the assembly of decamidine on a SAM of mercaptohexadecanoic acid in a pH 9 borate buffer revealed domains of similar size to that of the underlying acid SAM (20-30 nm), but less densely packed. By acidifying the solution, the second layer was destabilized and a very smooth layer with few defects appeared. Further acidification to pH 3 also destabilized the first layer.     

Fractal-like scaling in liquid crystals. Odd-even effect of scaling exponents in the 4,4'-di-n-alkyloxyazoxybenzene homologous series

A power law behaviour of dielectric loss versus frequency has been found in members of the 4,4'-di-n-alkyloxyazoxybenzene series. For each member the scaling of the data with respect to v/v_max is possible in the liquid-crystalline temperature range; this suggests a fractal-like geometry in nematic and smectic phases. The values of the scaling exponents, indicating a cooperativity of molecular reorientation around the short axes, exhibit an odd-even effect.     

Odd-even effects in the alignment of ferroelectric liquid crystals

Nine different poly-n-alkylterphthalamides have been synthesized, and their properties as alignment layers for chiral smectic C liquid crystals have been determined. Poly-n-hexylterphthalamide is found to be particularly well suited for one of the room temperature mixtures used in this study. The contrast between relaxed states shows an odd-even effect that can probably be explained by the crystal form of the different polymers. The tilt angle found between the relaxed states depends only on the polymer used, not on the inherent tilt of the chiral smectic C phases.     

Odd-even effects in the alignment of ferroelectric liquid crystals

Nine different poly-n-alkylterphthalamides have been synthesized, and their properties as alignment layers for chiral smectic C liquid crystals have been determined. Poly-n-hexylterphthalamide is found to be particularly well suited for one of the room temperature mixtures used in this study. The contrast between relaxed states shows an odd-even effect that can probably be explained by the crystal form of the different polymers. The tilt angle found between the relaxed states depends only on the polymer used, not on the inherent tilt of the chiral smectic C phases.     

Odd-even effects in the thermotropic and optical properties of three series of chiral twin liquid crystals

Three series of novel chiral twin liquid crystals consisting of a cholesteryl and a 4'-cyanobiphenyl-4-yloxy group (III) or a 4-cyanophenoxy group (IV) and of a dihydro-cholesteryl and a 4'-cyanobiphenyl-4-yloxy group (V) connected by an alkanoate spacer with a varying number (1-7 and 10) of methylene units were synthesized and their mesogenic properties investigated. Strong odd-even effects were observed as a function of spacer length for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phase. The compounds with an even number of methylene units have a smaller pitch than the compounds with an odd number of methylene units. Replacement of the cholesteryl group by a dihydrocholesteryl group results in an larger pitch, whereas replacement of the 4'-cyanobiphenyl-4-yloxy group by a 4-cyanophenoxy group gives a smaller pitch.     

Effect of the linking unit on the twist-bend nematic phase in liquid crystal dimers: a comparative study of two homologous series of methylene- and ether-linked dimers

ABSTRACT

Two series of bimesogens with phenyl benzoate mesogenic units were prepared: one series having a heptamethylene spacer and the other a pentamethylene spacer with two ether-linking groups. These materials were prepared to provide experimental backing to the widely held hypothesis that methylene-linked bimesogens are more likely to exhibit the twist-bend nematic mesophase than their ether-linked counterparts. Several of the methylene-linked materials exhibited nematic and N_TB mesophases, whereas the analogous ether-linked materials gave only nematic phases albeit with significantly higher clearing points. Virtual N–N_TB transition temperatures for both methylene- and ether-linked bimesogens were extrapolated by constructing binary phase diagrams with the well-studied twist-bend material CB9CB. Contrary to our expectations these virtual transition temperatures were in most cases higher for the ether-linked bimesogens than in the analogous methylene compounds, this runs counter to reported theories and hypotheses that the incorporation of ether-linking groups should serve to destabilise the N_TB phase.     

H-shaped liquid crystals inducing nematic order in the isotropic liquid

ABSTRACT

We prepared a homologous series of H-shaped liquid crystals I-n and investigated their phase transition properties using optical microscopy and differential scanning calorimetry. All the compounds exhibited a nematic phase at room temperature. The phase transition behaviour is explained in terms of molecular shape anisotropy. Furthermore, those compounds were found to exhibit electro-optical switching in the isotropic liquid in the vicinity of the nematic–isotropic liquid transition, indicating that the microscopic nematic order with a certain coherence length of the molecules exists in the optically isotropic temperature range.     

Odd-even effects in the phase transition behaviour of novel U-shaped liquid crystals

A homologous series of U-shaped dimeric liquid crystals in which two mesogenic groups are connected via catechol has been prepared and the effects of terminal alkyl chains, alkyl spacers and core structures on the transition properties investigated by means of optical microscopy and differential scanning calorimetry. The phase sequence exhibits a pronounced odd-even effect as the length and parity of the spacers is varied, in which the even members favour the nematic and smectic C phases, whereas the odd members favour the nematic and smectic A phases. We discuss the transition behaviour of the U-shaped compounds in terms of molecular shape.     

Odd-even effects in the thermotropic and optical properties of chiral triplet liquid crystals

A series of chiral triplet liquid crystalline compounds with a central biphenyl group to which two dihydrocholesteryl groups are attached via two flexible alkanoate spacers has been prepared and investigated. As a function of spacer length, these compounds show strong odd-even effects for the phase transition temperatures, the corresponding entropy changes and the selective reflection wavelengths associated with the chiral nematic phases. Asymmetrical compounds with one odd and one even number of methylene groups in their spacers have properties intermediate between those of symmetrical compounds with two odd or two even spacers.     

Effect of terminal aryl rings of liquid crystal trimers on liquid crystalline properties

Liquid crystalline properties of five series of biphenyl derivatives connected with p-nitrophenyl, phenyl, 2-naphthyl, 9-phenathryl and bromo as the terminal groups via flexible spacers were studied. All the compounds exhibited nematic phase. The nematic–isotropic or isotropic–nematic transition properties resulted in an odd–even effect as the length of the spacers was varied, in which the even members exhibited higher values, for the compounds with p-nitrophenyl, phenyl, 2-naphthyl and 9-phenanthryl groups, while the transitional properties of the compounds substituted with bromo atom as the terminal group instead of the aryl groups did not show such odd–even effect. These results suggest that the aryl rings, such as p-nitrophenyl, phenyl, 2-naphthyl and 9-phenanthryl groups, play a role of mesogen while the terminal bromo atom does not. The effect of the terminal aryl rings on the liquid crystalline properties will be reported.     

Electroconvection characterization of guest-host nematic liquid crystals for dynamic scattering mode applications

ABSTRACT

The threshold voltage for electro-hydrodynamic convection (EHC) in nematic liquid crystals in the low-frequency convective regime is examined. The addition of dopant species is known to strongly effect the frequency response of this regime. In this work, we investigate the effect of adding optically anisotropic species on the low-frequency threshold, and, demonstrate that certain dichroic dyes have a marked effect. We have measured electrical transport properties of suitably doped liquid crystal mixtures in order to make a comparison with theoretical predictions based upon linear stability considerations. Our results show satisfactory agreement with these predictions. Our findings indicate future directions for new applications of dynamic scattering mode technologies with more accessible operating parameters previously known.     

Solvent effect on the synthesis of clarithromycin: A molecular dynamics study

Clarithromycin (6-O-methylerythromycin A) is a 14-membered macrolide antibiotic which is active in vitro against clinically important gram-positive and gram-negative bacteria. The selectivity of the methylation of the C-6 OH group is studied on erythromycin A derivatives. To understand the effect of the solvent on the methylation process, detailed molecular dynamics (MD) simulations are performed in pure DMSO, pure THF and DMSO:THF (1:1) mixture by using the anions at the C-6, C-11 and C-12 positions of 2',4"-[O-bis(TMS)]erythromycin A 9-[O-(dimethylthexylsilyl)oxime] under the assumption that the anions are stable on the sub-nanosecond time scale. The conformations of the anions are not affected by the presence of the solvent mixture. The radial distribution functions are computed for the distribution of different solvent molecules around the 'O-' of the anions. At distances shorter than 5 A, DMSO molecules are found to cluster around the C-11 anion, whereas the anion at the C-12 position is surrounded by the THF molecules. The anion at the C-6 position is not blocked by the solvent molecules. The results are consistent with the experimental finding that the methylation yield at the latter position is increased in the presence of a DMSO:THF (1:1) solvent mixture. Thus, the effect of the solvent in enhancing the yield during the synthesis is not by changing the conformational properties of the anions, but rather by creating a suitable environment for methylation at the C-6 position.     

Electric field effects on the performance of a candidate multipole molecular switch: a quantum computational study

Structural and electronic responses of the organic molecule di(4-nitro-2-methylenamine phenyl) diazene a candidate molecular switch, as an active device in a nanoelectronic circuit, to the external electric fields with strengths 5 x 10(-4) - 1.8 x 10(-2) a.u. included explicitly in the Hamiltonian are studied using B3LYP/6-31G* method. This study shows that thermodynamic formation functions are not affected significantly by the applied field. Electronic spatial extent show a negligibly small change (<2%) over the studied range of the electric field strength. Calculated electric dipole moments show significant sensitivity to the external electric field, which result consequently in much stronger interactions with the electrodes (poles) of the mother nanoelectronic circuit at higher electric field strengths. Natural bond orbital atomic charges analysis shows different field effects on different atoms depending on their positions with respect to the direction of the field. The applied field increases HOMO, LUMO, and the Fermi level energies; however, decreases the HOMO-LUMO gap (HLG) values. Results of this study show that it is possible to control field-induced charge redistribution over the molecule by using push-pull effects of different substitution via their connection points to the extended pi-system.     

The oxadiazole as a structural fragment in liquid crystals

This paper examines in some detail the effect of introducing the oxadiazole fragment into the molecular core of liquid crystals on their physico-chemical properties. The results of this study are compared with those obtained for other molecular fragments, and are rationalized in terms of existing theories.     

Anchoring effects on the electrically controlled optical band gap in twisted photonic liquid crystals

We study a one-dimensional twisted photonic liquid crystal (TPLC), consisting of various nematic liquid crystal cells adopting a twisted configuration intercalated by isotropic dielectric layers, submitted to a dc electric field (E^dc ) aligned along the periodicity axis. We write the corresponding Euler–Lagrange equations describing the nematic layer configuration. By assuming arbitrary anchoring quasi-planar boundary conditions, we calculate the equilibrium textures for the nematic, parametrized by the two types of strength of its interaction (polar and azimuthal) with the plane walls. We write the electromagnetic equations in a 4?×?4 matrix representation and using the transfer matrix formalism, we obtain the transmittance and reflectance coefficients for normal incidence as functions of the external electric field and anchoring strengths. We have observed a remarkable dependence of the electric field on the transmission and reflection spectra in opening and closing band gaps.     

Synthesis and mesomorphic behavior of a new series of side-chain liquid crystalline polymethacrylates bearing 2,6-naphthylene-based mesogen

A new homologous series of liquid crystalline (LC) polymethacrylates, the poly[ω-(2-phenoxycarbonyl-naphthalene-6-yloxy)alkyl methacrylate]s, in which the length of the alkyl spacer is varied between 3 and 12 methylene units, have been synthesized by free-radical polymerization. The prepared polymers were studied by IR and ¹H-NMR spectroscopy, viscosity measurements, differential scanning calorimetry (DSC), and polarized microscopy. Polymers with 3 or 4 methylene units spacer were glassy in nature, whereas those with 5–12 methylene units spacer exhibited smectic behavior. The glass transition temperatures decrease on increasing spacer length. The isotropization temperatures and the corresponding transition entropies showed an odd–even effect, with the odd members exhibiting the higher values. This effect attenuates on increasing spacer length. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2391–2399, 1999     

Substituent effect on ionisation potential in a series of related molecules: A theoretical study in a molecular orbital framework

The effect of replacing the hydrogen atoms in thioformaldehyde by halogen atoms (F, Cl) on the ionisation potential of the non-bonding electron is analysed by using the Hellman-Feynman theorem, regarding the nuclear charge of the substituent as a parameter in the many-electron Hamiltonian. The trends predicted by our theory nicely agree with the relevant ionisation potentials computed either by applying Koopmans’ theorem or by the ΔE _SCF method. For the carbonyls, avaible experimental data indicate the reliability of our prediction.     

Synthesis and liquid-crystalline properties of a homologous series of antiferroelectric liquid crystals and their respective racemates

New chiral, as well as racemic 1-methylalkyl terephthalidene-bis-aminocinnamates have been synthesized and their liquid-crystalline properties investigated. Generally, the chiral compounds possess the antiferroelectric S*_o phase, whereas in the homologous series of racemates, the appearance of the S_o phase depends on the length of the alkyl chain. For racemic compounds derived from 2-alkanols with an odd number of carbon atoms, a S_o phase is generally observed. On the other hand when the number of carbon atoms is even, a S_c phase occurs. Moreover, in racemic 1-methylbutyl terephthalidene-bis-aminocinnamate, a S_c-S_o-S_c reentrant phase sequence has been established.     

Computing the (1)H NMR spectrum of a bulk ionic liquid from snapshots of car-parrinello molecular dynamics simulations.

We have investigated the performance of several computational protocols in predicting the NMR spectrum of a molecular ion in a complex liquid phase such as an ionic liquid. To do this, we computed the proton NMR chemical shifts of the 1-ethyl-3-methylimidazolium cation [emim](+) in [emim][Cl]. Environmental effects on the imidazolium ring proton chemical shifts are quite significant and must be taken into account explicitly. Calculations performed on the isolated imidazolium cation as well as on the [emim][Cl] ion pair grossly fail to reproduce the correct spacing between proton signals. In contrast, calculations performed on clusters extracted from the trajectory of a Car-Parrinello molecular dynamics simulation yield very good results.